Although embryonic stem cells (ESCs) have been established from mice since 1981, attempts to derive their counterparts from various other mammals, including rats, have not succeeded. Recently, pluripotent stem cells were derived from the post-implantation egg cylinder stage Epiblasts of mouse and rat (Brons et al., Nature 448, 191-195 (2007); Tesar et al., Nature 448, 196-199 (2007)). These novel stem cells were named Epiblast stem cells (EpiSCs). EpiSCs seem to correspond very closely to human embryonic stem cells (hESCs) in the colony morphology and culture/signaling requirements for maintaining pluripotency, but exhibit a range of significant phenotypic and signaling response differences from the mouse ES cells (mESCs).
Leukemia inhibitory factor (LIF) is essential for maintaining the pluripotency of mESCs in the presence of serum through JAK-STAT3 pathway (Niwa et al., Genes Dev 12, 2048-2060 (1998)). However, in serum-free medium, BMP4 is also required, together with LIF, to sustain mESC self-renewal by inducing inhibitor of differentiation (Id) protein expression (Ying et al., Cell 115, 281-292 (2003)) and inhibiting ERK activation (Qi et al., Proc Natl Acad Sci USA 101, 6027-6032 (2005)). In contrast to mESCs, LIF cannot support EpiSCs/hESCs, which typically require basic fibroblast growth factor (bFGF)/Activin A for long term self-renewal. Undifferentiated hESCs display high-level basal activity of ERK through bFGF signaling (Dvorak et al., Stem Cells 23, 1200-1211 (2005)). BMP4 doesn't support EpiSC/hESC self-renewal either, but instead induces EpiSC/hESC to differentiate into trophoblasts or primitive endoderm (Brons et al., Nature 448, 191-195 (2007); Tesar et al., Nature 448, 196-199 (2007); Xu et al., Nat Biotechnol 20, 1261-1264 (2002)). In addition to bFGF, Activin A/Nodal signaling has been shown to support the undifferentiated state of hESCs/EpiSCs (Brons et al., Nature 448, 191-195 (2007); Sato et al., Dev Biol 260, 404-413 (2003); Tesar et al., Nature 448, 196-199 (2007)), while is dispensable for mESCs. These results strongly support the notion that EpiSCs and hESCs are intrinsically similar and raise an attractive hypothesis that mESCs and EpiSCs/hESCs represent two distinct pluripotent states: the mESC-like state representing the pre-implantation inner cell mass (ICM) and EpiSC-like state representing later Epiblast cells, respectively.
mESCs can be usually derived from certain mouse strains using feeder layer based cell culture conditions (Martin, G. R., Proc Natl Acad Sci USA 78, 7634-7638 (1981)). However, it has been proven difficult to derive authentic ES cells from rats under similar conditions. Establishments of rat ESC-like cells have been reported (Demers et al., Cloning Stem Cells 9, 512-522 (2007); Ruhnke et al., Stem Cells 21, 428-436 (2003); Schulze et al., Methods Mol Biol 329, 45-58 (2006); Ueda et al., PLoS ONE 3, e2800 (2008)), but these cells either could not be stably maintained or lacked true in vivo pluripotency (e.g. fail to form teratoma or no/little contribution to chimerism). Similarly, although (in vitro) pluripotent rat EpiSCs had been derived, both rat and mouse EpiSCs show little or no ability to be reincorporated into the pre-implantation embryo and contribute to chimaeras (Brons et al., Nature 448, 191-195 (2007); Tesar et al., Nature 448, 196-199 (2007)).
Recently, induced pluripotent stem cells (iPSCs) generated from both mouse and human somatic cells by defined genetic transduction have attracted enormous interests (Dimos et al., Science 321, 1218-1221 (2008); Han, J., and Sidhu, K. S. Curr Stem Cell Res Ther 3, 66-74 (2008); Takahashi et al., Cell 131, 861-872 (2007); Takahashi, K., and Yamanaka, S., Cell 126, 663-676 (2006); Yu et al., Science 318, 1917-1920 (2007)).